i) Field of the Invention
This invention relates to macromers derived from the thermal degradation of poly(3-hydroxybutyrate), poly(3-hydroxyvalerate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate); a process for their preparation, homopolymers and copolymers derived from the macromers the use of such homopolymers and copolymers to provide product having amphiphilic and biocompatible properties, for example, in drug delivery systems, polymer surfactants and biocompatible adjuvants.
ii) Description of Prior Art
Poly(3-hydroxybutyrate), PHB; poly(3-hydroxyvalerate), PHV; and poly(3-hydroxybutyrate-co-3-hydroxy-valerate), PHB/V, are aliphatic thermoplastic polyesters[1,2,3] of formulae Ia, Ib and Ic, respectively: 
Where n and m are integers indicating the number of repeat units, and p and q are mole % of each monomer randomly distributed [3]. Typically, n is an integer of 10 to 10,000,000, preferably 100 to 1,000,000; m is an integer of 10 to 200,000 preferably 60,000; p and q are each 0 to 100, and the summation p + q is 100.
PHB and PHB/V are poly(hydroxyalkanoates), PHAs, known as bacterial carbon and energy storage materials. They are biodegradable and biocompatible polymers produced by a large number of bacteria such as Alcaligenes eutrophus[3,4,5], now called Ralstonia eutropha. PHB is brittle, which reduces its potential industrial applications, but the incorporation of 3-hydroxyvalerate repeat units has been shown to improve flexibility in the resulting copolymer by reducing crystallinity[3,6]. PHB and PHB/V are crystalline polymers, with melting points (Tm) of ca. 180xc2x0 C. for PHB and a range of 70 to ca 180xc2x0 C. for the PHB/V copolymers[7,8].
PHB, PHV and PHB/V are xcex2-polyesters, i.e. their xcex2-carbons are substituted, which makes them thermally unstable at temperatures higher than their melting point[3].
It is an object of this invention to provide macromers derived from PHB, PHV and PHB/V, respectively.
It is a further object of this invention to provide a process for producing the aforementioned macromers.
It is another object of this invention to provide families of macromers which differ in end group functionality and/or in molecular weight.
It is still another object of this invention to provide macromers derived from the thermal degradation of poly(3-hydroxyalkanoate)s.
It is yet another object of this invention to provide homopolymers of the macromers.
It is still another object of this invention to provide copolymers of the macromers with one or more comonomers.
In accordance with one aspect of the invention, there is provided a macromer derived from thermal degradation of poly(3-hydroxybutyrate), poly(3-hydroxyvalerate), or poly(3-hydroxybutyrate-co-3-hydroxyvalerate). In accordance with another aspect of the invention, there is provided a process of producing a macromer comprising thermally degrading poly(3-hydroxybutyrate), poly(3-hydroxyvalerate), or poly(3-hydroxybutyrate-co-3-hydroxyvalerate).
In accordance with other aspects of the invention, there is provided a homopolymer of the macromer of the invention; a copolymer of the macromer of the invention and a comonomer; and a block copolymer of a macromer of the invention, and a block comonomer.
The present invention focuses on macromers of PHB, PHV and PHB/V, which are macromolecules of small size or length, typically between 1000 and 6000 g/mol, and their preparation by thermal degradation at a constant temperature.
Suitably, the thermal degradation is carried out at a temperature of 180xc2x0 C. to 220xc2x0 C.
In particular embodiments, PHB and copolymers of PHB/V having up to 21 mol% of hydroxyvalerate were treated at two different temperatures, 190-192xc2x0 C. and 200-202xc2x0 C., for a reaction time of 3.5 hrs, and then fractionated. The products obtained were then characterized by proton nuclear magnetic resonance spectroscopy (1H-NMR) and gel permeation chromatography (GPC). The copolymers were also degraded at 190+/xe2x88x921xc2x0 C. for reaction times of 3.5, 5, and 7 hrs, and the crude products were analysed by 1H-NMR and GPC. The homopolymers PHB and PHV were analyzed also by positive fast atom bombardment mass spectrometry (FAB+-MS). The resulting macromers can be used as monomers for polymerization, for example, homopolymerization or copolymerization with one or more comonomers. Thermal degradation produces low molecular weight PHB and PHB/V containing one unsaturated end (Scheme 1). At moderately high temperatures, the reaction proceeds by a random scission cis-elimination mechanism having a six-membered ring ester intermediate[8,9,10], which is shown on Scheme 2. 
In Scheme 1, the methyl sidegroup can be replaced by ethyl; the integer n is 10 to 10,000,000, preferably 100 to 1,000,000 when the terminal sidegroup is methyl.
The scale-up experiments were performed at 201xc2x0 C. (average value), 3 hours and gave similar results to lab-scale reactions.
Polymer architecture is controlled by a wide variety of catalysts and synthesis strategies. The use of macromers of the invention to make architectures involving PHB, PHV and PHB/V blocks is enabled by the availability of alpha and omega functionally terminated low molecular weight poly(3-hydroxyalkanoates). The structure of the PHB polyester may be represented as: 
where n is an integer indicating the number of repeat units. Suitably n is 10 to 10,000,000, preferably 100 to 1,000,000.
Specifically, the macromer can be used to make block and comb-like polymers and other architectures with amphiphilic and biocompatible properties for applications such as drug delivery, polymer surfactant, biocompatible adjuvant. The following molecular models A to E show some of the PHB macromers which can be produced: 
The models A to E may be generally represented by formula II: 
In formula II Rxe2x80x2 is a PHB macromer end group providing a carboxyl or carboxylate alpha end; and R provides a propenyl omega end (Models A and B) or a beta hydroxyl or methoxide end (Models C, D and E). The designation 
in formula II identifies repeating continuity of the PHB as shown in II above.
Various controlled chemical chain scissions and end group reactions can be used to create the above models A to E, including alkaline and acid hydrolysis as well as pyrolysis. Under certain conditions the reactions lead to narrow molecular weight macromers with yields of 70-90%.
Typically the macromers of the invention have a number average molecular weight of 1,000 to 6,000 g/mol.
Two major ways of polymerization from the macromers are possible, using the xcex2-alkyl acrylate end group, or the carboxylic acid end group.
Polymerization by the unsaturated end group:
The particularity of the olefin end group is the xcex2-substitution with a methyl, and its predominantly trans configuration.
The following equation represents the formation of a comb polymer from the xcex2-methylacrylate macromer below by chemical linkage of macromers via a polymerization reaction at the double bond end, in which R is methyl as shown in models A and B but may also be ethyl, in the case of macromers from PHB/V or PHV or other terminal moiety as shown in models C, D and E. R3 is the appropriate alpha moiety, such as indicated for formula II in the case of PHB. 
The Group Transfer polymerization may be employed to polymerize the macromers. Ute et al. reported the polymerization of trans-methylcrotonate 1 via this method, using 1-methoxy-1-(trimethylsiloxy)-2-methyl-1-propene 2 with catalytic amounts of HgI2 and (CH3)3 SiI in CH2Cl2. [Ute et al. Polymer Journal 1997, 29, 11, 957-958, also 1999, 31, 2, 177-183]. 
Polymerization by the carboxylic acid end group:
A possible approach involves the modification of the carboxylic acid end group to increase the reactivity of the macromers towards polymerization. The carboxylic acid ends could be reacted with 2-hydroxyethyl methacrylate (HEMA) to produce macromers with methacrylate-type ends on one side, via an esterification with, for example, 1.3-dicyclohexylcarbodiimide (DCC) and 4-(dimethylamino) pyridyne (DMAP), or by reaction of HEMA on the acyl 
chloride of the macromers. The reaction scheme is the following:
As HEMA is well known to polymerize easily through free radical polymerization, the transformed macromers are expected to polymerize with free radicals. The polymerization scheme is shown below. 
Copolymers of HEMA and the macromers may be obtained from the same method, by using both monomers, in a suitable solvent accommodating the hydrophilicity of poly(HEMA) and the hydrophobicity of the macromers of PHB.
A controlled free radical polymerization method, such as the atom transfer radical polymerization (ATRP) may be employed. A few examples of polymerization of HEMA by ATRP have been reported (Matyjaszewski et al. Macromolecules 1999, 32, 5772-5776, also Armes et al. Macromolecules 2001, 34, 3155-3158).
The preparation of block copolymers may be achieved by coupling the macromers with monomethoxy polyethylene glycol (PEG) employing the procedure of (R. H. Marchessault and G. E. Yu, Polymer Preprints p. 527 40, No 1, 1999), where the macromer was prepared by heterogeneous hydrolysis of PHB with methanolic sodium methoxide. As the comonomer or coblock for reaction with the macromers of the invention there may be employed those monomers or blocks which have a polymer chain with a functional end group which reacts with the COOH end group of the macromer, for example: amino, hydroxyl or isocyanate functional end groups. By way of example, the comonomer or coblock may be a polyethylene glycol moiety having a terminal amino, hydroxyl or isocyanate group. The amino groups, hydroxyl groups and isocyanate groups form amide, ester and urethane linkages respectively, on reaction with the carboxylic acid groups of the macromers.
By way of further example, amphiphilic block copolymers may be formed from macromers of the invention with propylene oxide and ethylene oxide in appropriate monomer ratios to produce a water soluble polymer. Similarly a ring carbohydrate such as a cycloamylose could be a suitable water soluble moiety.
In general, polymer size for homopolymer, copolymer or block copolymer product will be dictated by the preference for water solubility; the choice and proportions of comonomers and block comonomers will also be dictated by the preference for water solubility in the resulting polymer.
The carboxylic acid group in the macromers of the invention may also be grafted to soluble polymers or active surface containing accessible functional groups employing the techniques and procedures described in M. Yalpani et al, Macromolecules 24, 6046 (1991) and G. Yu et al, Macromolecules 32, 518 (1999) the teachings of which are incorporated herein by reference.
In general, comonomers for producing copolymers of the macromer of the invention include vinyl monomers, for example, acrylates. Block comonomer for producing block copolymers of the macromers of the invention include polyethylene glycol, polypropylene glycol, polyurethane and vinyl monomers including acrylates.